1. Field of the Invention
This invention relates to a noise cancellation system, and to an earphone for use with such a system.
2. Description of the Related Art
It is known to provide a noise cancellation system, for use with a sound-reproducing device such as an earphone. The sound-reproducing device includes a speaker, for receiving electrical signals representing a wanted sound, such as music or speech, from a portable music player, telephone handset, or the like. The noise cancellation system includes a microphone provided on the sound-reproducing device, to generate an electrical signal representing ambient noise. This ambient noise signal is then applied to signal processing circuitry to generate a noise cancellation signal, and the noise cancellation signal is applied to the speaker.
The purpose of the signal processing circuitry is to generate a noise cancellation signal that, when applied to the speaker, produces a sound that is equal in magnitude but opposite in phase to the ambient sounds reaching the user's ear. If this can be achieved, destructive interference will have the effect of reducing the noise that can be heard by the user.
In order to achieve this, it is known, for example from GB-2441835A, that the signal processing circuitry needs to apply frequency-selective filtering to the ambient noise signal, and that this frequency-selective filtering needs to take account of the frequency-dependent amplitude and phase characteristics of: the response of the noise microphone; any electronic amplification in the signal processing circuitry; and the response of the speaker. These characteristics are generally relatively stable for any given individual earphone device and, subject to manufacturing tolerances, they can be determined for any model of earphone.
In addition, however, the frequency-selective filtering needs to take account of two further factors, namely the frequency-dependent amplitude and phase characteristics of the acoustic path from the surroundings into the ear of the user, and the phase and frequency response of the acoustic path from the speaker to the ear of the user. These are both dependent on the leakage characteristics of the earphone, that is, the leakage in the coupling of the earphone to the ear of the wearer.
It is known that the frequency-dependent characteristics of the leakage path can vary widely, depending on how the sound-reproducing device interacts with the ear of the user. More specifically, one important factor is the area of the leakage, which affects both the amplitude and phase of all signals perceived by the ear. For example, in the case of an earphone that is intended to be worn within the outer ear of the user, the frequency-dependent leakage characteristics will depend on the exact shape of the user's ear, and on how tightly the earphone is pushed into the ear.
This has the effect that it is difficult to perform frequency-selective filtering that is sufficiently representative of the frequency-dependent amplitude and phase leakage characteristics.